1. Field of the Invention
The present invention relates to a laser interferometer for accurately measuring a distance or length of a moving member, and in particular, to such a laser interferometer which is capable of measuring a distance of or length traveled by a moving member accurately and has a simple structure.
2. Description of the Prior Art
A prior art laser interferometer is described, for example, in a publication "Laser and its Application &lt;additional edition&gt;, published on June 10, 1982, by NISSANPO Publishing Co., Ltd., pages 146-149.
In this prior art interferometric measuring machine, the so-called Michelson interferometer is applied to a laser interferometric measuring machine. As shown in FIG. 2.2 in the cited publication, it is composed of a laser device; an inverted telescope system; an interference system having a beam splitter, two fixed mirrors, a photoelectric tube, and a fringe counter; and a movable mirror fixed to a moving member. A beam of light emitted from the laser passes through the inverted telescope and enters the interferometer, and further travels towards the movable mirror through the beam splitter. The light reflected from the movable mirror is further reflected by the fixed mirror disposed in the vicinity of the beam splitter. The reflected light is again reflected by the movable mirror and returns to the beam splitter to enter into the photoelectric tube. In the photoelectric tube, the reflected light joins with another reflected light from the fixed mirror to produce interference. By counting the number of bright bands or fringes by a fringe counter, the length of the moving member is measured.
However, in the aforementioned prior art laser interferometer, a great number of parts are used and the structure is complicated. It is expensive since the precision of the parts used is very high: high skill and technique are required to register the optical axes of the respective parts and to adjust the optical paths. Further the S/N (signal to noise) ratio is small since the interference fringes are detected as a single dimensional signal representing bright and dark and the measuring process of the interference light is complicated.